Titanium alloys are advanced lightweight materials, indispensable for many critical applications^1,2. The mainstay of the titanium industry is the α–β titanium alloys, which are formulated through alloying additions that stabilize the α and β phases^3–5. Our work focuses on harnessing two of the most powerful stabilizing elements and strengtheners for α–β titanium alloys, oxygen and iron^1–5, which are readily abundant. However, the embrittling effect of oxygen^6,7, described colloquially as ‘the kryptonite to titanium’⁸, and the microsegregation of iron⁹ have hindered their combination for the development of strong and ductile α–β titanium–oxygen–iron alloys. Here we integrate alloy design with additive manufacturing (AM) process design to demonstrate a series of titanium–oxygen–iron compositions that exhibit outstanding tensile properties. We explain the atomic-scale origins of these properties using various characterization techniques. The abundance of oxygen and iron and the process simplicity for net-shape or near-net-shape manufacturing by AM make these α–β titanium–oxygen–iron alloys attractive for a diverse range of applications. Furthermore, they offer promise for industrial-scale use of off-grade sponge titanium or sponge titanium–oxygen–iron^10,11, an industrial waste product at present. The economic and environmental potential to reduce the carbon footprint of the energy-intensive sponge titanium production¹² is substantial.

An FeCoCrNi high-entropy alloy (HEA) was deformed at ambient temperature and cryogenic temperatures down to 4.2 K. Phase transformation from a face-centered cubic (FCC) structure to a hexagonal close-packed (HCP) structure occurred during cryogenic deformation. Lowering the temperature promotes the transformation. Atomic-scale structural characterisation suggested that the formation of the HCP structure was achieved via the glide of Shockley partial dislocations on every other plane. Due to the kinetic limitation, the occurrence of this transformation was limited even though theoretical investigations predicted lower free energy of the HCP phase than that of the FCC phase in the HEA. GRAPHICAL ABSTRACT IMPACT STATEMENT Atomic-scale FCC–to–HCP transformation was revealed in an FeCoCrNi high-entropy alloy during cryogenic-temperature deformation. The transformation was restricted due to the kinetic limitation.

In situ TEM experiments reveal the crystalline-to-amorphous phase transformation in an ultrafine-grained Cantor alloy.

Titanium alloys can experience a cooling-induced phase transformation from a body-centred cubic phase into a hexagonal close-packed phase which occurs in 12 crystallographically equivalent variants. Among them, variant selection II, 60°/ $〈1\overline{2}10〉$, is very close to the orientation of $\left\{10\overline{1}1\right\}〈1\overline{2}10〉$ twins (57.42°/ $〈1\overline{2}10〉$). We propose that the cyclic thermal loading during additive manufacturing introduces large thermal stresses at high temperature, enabling grain reorientation that transforms the 60°/ $〈1\overline{2}10〉$ variant boundaries into the more energetically stable 57.42°/ $〈1\overline{2}10〉$ twin boundaries. This transformation twinning phenomenon follows a strain accommodation mechanism and the resulting boundary structure benefits the mechanical properties and thermal stability of titanium alloys. GRAPHICAL ABSTRACT IMPACT STATEMENT A new twinning mechanism, transformation twinning, was discovered in a Ti–6Al–4V alloy fabricated by selective laser melting. The resulting high density of transformation twins impact the global mechanical properties significantly.